Mammals developed out of mammal-like reptiles, the therapsids. While there was no doubt that these were reptiles, they displayed characteristics that are only to be found in mammals today. When the dinosaurs were in their heyday, 200 million years ago, the first animals that were clearly mammals survived this age as small creatures the size of rats or mice.
Today, mammals come in many different shapes and sizes. All of them, however, share the typical characteristics of mammals.

The skin of the warm-blooded terrestrial vertebrates has a covering of hair, which has disappeared again in individual groups due to their subsequent adaptation to life in the water. In addition to hairs, the skin also forms different glands: sebaceous glands, sweat glands, scent glands and mammary glands. Only female animals develop mammary glands, which they use to feed their young. Nails, claws or hooves form at the end of the fingers and toes. The teeth are adapted to the corresponding eating patterns and are completely replaced once in the course of the mammal's life. Almost all mammals have seven cervical vertebrae. The individual sensory organs, and particularly the brain, have undergone a high level of development.